Many types of devices must be calibrated, at least at selected intervals, to ensure their proper operation. A VCO (voltage-controlled oscillator) coupled in a phase-locked relationship with a reference signal is exemplary of such a device.
When, e.g., the VCO forms a portion of a PLL (phase-locked loop) circuit, the oscillation frequency of oscillating signals generated by the VCO are locked to that of a reference signal to which the VCO is operably responsive.
Many types of radio communication apparatus utilize VCOs coupled in PLL circuits. Oscillating signals formed by the VCO are used to form transmit signals which are transmitted by transmitter apparatus. And, oscillating signals generated by the VCOs of receiver apparatus, for instance, are used in the reception of receive signals. Acceptable frequency stabilities of the oscillating signals generated by the VCOs are required for proper operation of the radio communication apparatus.
A radio base station operable in a cellular communication system is exemplary of radio communication apparatus which utilizes a VCO coupled in a PLL circuit Acceptable frequency stability of oscillating signals generated by the VCO is required so that downlink signals generated by the radio base station are properly transmitted to a mobile terminal without interfering with other concurrently-transmitted downlink signals. Acceptable levels of frequency stability are similarly required to permit the radio base station to properly receive uplink signals transmitted by mobile terminals to the radio base station.
Operational specifications promulgated by various standard-setting bodies set forth, inter alia, frequency stability requirements within which operation of cellular communication apparatus must comply. Operational specifications for the GSM, PCS 1900, and DCS 1800 mobile cellular radio communication systems all set forth stringent timing accuracies to ensure that radio base stations operable in such systems at least generate signals which exhibit acceptable frequency stability levels.
Compliance with the required timing accuracies set forth in an appropriate operational specification required at a radio base station is assured by using a PI-regulated phase-locked-loop (PLL) circuit. In such a circuit, a VCO is locked to a reference signal of high frequency stability. For instance, a PCM clock signal is sometimes utilized to form the reference signal applied to the radio base station. The PCM clock signal is an 8 kHz reference signal generated by a network operator in a well-controlled environment. ETSI G.823 and G.824 specifications set forth inter alia, allowable levels of jitter in a PCM clock signal. When the PCM clock signal exhibits characteristics which are at least as good as the signal requirements set forth in the appropriate or of such specifications, the radio base station can be operated in compliance with the operational specification promulgated by the appropriate standard-setting body.
The quality of the reference signal is measured in Stratum levels. The Stratum level of a PCM clock signal specifies maximal allowed frequency deviation of the reference signal. The aforementioned ETSI G.823 and G.824 specifications set forth frequency stability standards corresponding to a "Stratum-2" level.
Some networks, however, do not guarantee the reference signal, such as the PCM clock signal, to be always within the requirements set forth in the appropriate one of the ETSI G.823 and G.824 specifications.
A reference signal of a lesser-assured stability level is more inexpensively provided. In some networks, therefore, a reference signal, not guaranteed always to be of a Stratum-2 level is instead provided to the radio base stations. A reference signal of a Stratum-3 level is instead guaranteed to be provided. The reference signal provided by the network to the radio base stations is not assured always to be of a Stratum-2 level, but the reference signal is normally of a Stratum-2 frequency stability level but for intermittent periods. During such periods, the reference signal is of an inadequate, i.e., Stratum-3, frequency stability level. Such a reference signal, therefore, is of good long-term frequency stability characteristics, but is potentially of poor short-term frequency stability characteristics.
To ensure that the frequency stability standards required of operation of the radio base stations are met in such networks, some radio base stations include reference oscillators which generate reference signals of Stratum-2 quality. An OVCXO (oven voltage controlled crystal oscillator) is exemplary of a Stratum-2 oscillator.
An OVCXO, as well as some other types of Stratum-2 oscillators, exhibits short-term frequency stability, but is susceptible to long-term frequency drift caused by aging of the oscillator. Conventionally, such oscillators must be calibrated regularly. Calibration is typically carried out utilizing a manual procedure. Such a procedure is costly, particularly when large numbers of radio base stations of a radio communication system must all be regularly calibrated.
A manner by which the good long-term frequency stability characteristics of a PCM clock signal provided by the network can be used to correct for the aging of the Stratum-2 oscillator positioned at the radio base station would reduce the need to manually calibrate the Stratum-2 oscillator.
More generally, a manner by which to permit a device to be calibrated with a reference signal subject to short term disturbances would be advantageous.
It is in light of this background information related to calibration apparatus and methods that the significant improvements of the present invention have evolved.